The invention relates to an ultrasonic microscope having an optimum impedance matching layer for use.
It has long been known to observe the microscopic structure of a substance using ultrasonic waves in lieu of light rays. To this end, ultrasonic microscopes scan a specimen surface mechanically with ultra-high frequency-ultrasonic wave beams, convert the ultrasonic waves scattered by the specimen into electrical signals by concentrating the scattered waves and display the signals on a display plane of a cathode-ray tube in two dimensions so that a microscope image can be obtained. In construction, ultrasonic microscopes are divided into two types: the transmission type and the reflection type depending on how the ultrasonic waves are detected. In the transmission type, ultrasonic waves are transmitted through a specimen undergoing scattering or attenuation and are then detected. In the reflection type, ultrasonic waves are reflected by the difference in acoustic properties inside the specimen and are then detected.
In the accompanying drawings, FIG. 1 is a diagram explaining a principle of the reflection type ultrasonic microscope. As shown, a signal from a high frequency oscillator 1 is applied to a transmitting and receiving transducer 3 by a directional coupler 2. This signal is converted into ultrasonic waves and these waves are radiated from one surface of an ultrasonic condensing lens 4 which transmits and receives them. The lens 4 is composed of an ultrasonic propagation medium such as sapphire and is attached to the transducer 3, at the inside of the ultrasonic condensing lens 4. The other surface of the ultrasonic condensing lens 4 forms a spherical lens portion 4a opposite to which a specimen holding plate 5 is disposed. An acoustic field medium 6 composed of water is interposed between the ultrasonic condensing lens 4 and the holding plate 5 and a specimen 7 is mounted on the plate 5 at the focus of the spherical lens portion 4a. The holding plate 5 is moved in the X-Y directions by a scanning unit 8 which is controlled by a scanning circuit 9. The ultrasonic waves incident on the ultrasonic condensing lens 4 from the transducer 3 are focused on the specimen 7. The ultrasonic waves reflected by the specimen 7 are gathered by the condensing lens 4 and inverted into an electrical signal by the transducer 3 so that the electrical signal is fed through the directional coupler 2 to a display unit 10.
However, as shown in FIGS. 2 and 3, as a practical matter the signals appearing on the display unit 10 are not only a signal indicated by (c) which is reflected by the specimen 7 but also a signal (a) which is produced by leakage waves from the directional coupler 2 and the transducer 3 and a signal (b.sub.1) reflected by the boundary surface of the spherical lens portion 4a of the condensing lens 4, its second reflection signal (b.sub.2) thereby, its third reflection signal (b.sub.3), its fourth reflection signal (b.sub.4) and so on which are picked up as the electrical signals by the transducer 3.
This fact indicates that only a part of the generated ultrasonic energy is utilized effectively and also the increase of the reflected waves within the condensing lens 4 deteriorates the signal-to-noise ratio of the signal from the specimen 7.
It is to be noted that the occurrence of reflection at the boundary surface of the spherical lens portion 4a of the condensing lens 4 is due to the discontinuity between the acoustic impedances of the material of the condensing lens 4 and the acoustic field medium 6. It is well known that the prevention against the above-mentioned fact is to provide an impedance matching layer on the spherical lens portion 4a. The optimum acoustic impedance Z.sub.M for the matching layer is expressed by the following formula: Z.sub.M =.sqroot.Z.sub.1.Z.sub.2 where Z.sub.1 is an acoustic impedance of the condensing lens 4 and Z.sub.2 is the acoustic impedance of the acoustic field medium 6. It is possible to prevent the reflection at the boundary surface by selecting a material having the optimum acoustic impedance and forming the impedance matching layer of the thickness of .lambda./4 (.lambda. is a wavelength of the acoustic wave passing through the matching layer) between the spherical lens portion 4a and the acoustic field medium 6.
However, it is difficult in practice to obtain a material having the optimum acoustic impedance and therefore materials comparatively close to it are presently selected. By way of example, when sapphire is used as the material for the condensing lens 4 and water is used as the acoustic medium 6, the acoustic impedance of the optimum matching layer is 8.19.times.10.sup.6 kg.m.sup.-2.S.sup.-1. Such a material can not, however, be obtained as a simple substance. While SiO.sub.2 is used in practice as a material close to the value of the acoustic impedance as shown above, the acoustic impedance for SiO.sub.2 is 13.14.times.10.sup.6 kg.m.sup.-2.S.sup.-1, which is impossible to form a proper matching layer.